The effect of varying the percentage of organic modifier and the pH of the aqueous component in the eluent was investigated on drug-free plasma profiles using reversedphase HPLC. Two different organic modifiers were used (methanol and acetonitrile) on two columns with differing selectivities (C - 18 and CN). Thus three variables were involved at any time - the column type, the organic modifier and either the percentage organic modifier or the pH of the eluent. The samples were prepared by liquid-liquid extraction into a diethylether solvent. The resultant chromatograms were assessed in terms of the number of interfering peaks present above a certain value of peak height. Analysis of the results showed that the number of peaks depended strongly on the percentage of organic modifier and was generally inversely proportional to the pH. The effects were also significantly dependant on the type of column and organic modifier used. An additional ‘salting out’ procedure revealed that for particular salts, the number of interfering peaks was reduced.